The lithium battery, also called lithium secondary battery, is a battery that contains, for example, liquid, gel-state polymer, solid polymer, and polymer electrolyte and produces electric current by virtue of movement of lithium ions and in which polymers are used as active substances of positive and negative electrodes. A lithium secondary battery consists mainly of a positive electrode collectors (aluminum, nickel)/positive electrode active substance layer (metal oxide, carbon black, metal sulfide, electrolytic liquid, polyacrylonitrile or other polymer positive electrode materials)/electrolyte layer (propylene carbonate, ethylene carbonate, dimethyl carbonate, ethylene methyl carbonate or other carbonate based electrolytic liquids, lithium salt based inorganic solid electrolyte, gel electrolyte)/negative electrode active substance layer (lithium metal, alloy, carbon, electrolytic liquid, polyacrylonitrile or other polymer negative electrode materials)/negative electrode collectors (copper, nickel, stainless steel), and housing for their packaging. Being high in high volume efficiency and weight efficiency, lithium batteries in recent years have been used as small sized, large capacity power sources in a variety of products including personal computers, mobile devices (portable telephone, PDA or the like), video cameras, electric automobiles, energy storage batteries, robots, and satellites.
Exterior packages of lithium batteries include tubular or rectangular parallelepiped metal containers produced by press working of sheet metal, and bags of multi-layered film formed of outermost layer/aluminum/sealant layer. However, metal containers have rigid outer walls that define the shapes of batteries, and hard containers have to be designed to meet requirements of the batteries to be contained, leading to the problem of limitations on the size of the container used to contain the battery. Therefore, bags formed of multi-layered film have been more favored these days. Exterior packages of lithium batteries are required to have physical properties and functions such as moisture resistance, resistance to contents (resistance to electrolytic liquid or other compounds to be contained), and formability, and film materials presently in use to meet these requirement include polyamide film (for instance, see Japanese Unexamined Patent Publication (Kokai) No. 2006-236938). However, polyamide film is not sufficiently high in resistance to moisture and contents, and the polyamide material is likely to deteriorate when coming in contact with the contents during processing, resulting in increased needs for improvements. Studies are also being made concerning the use of polyester film (for instance, see Japanese Unexamined Patent Publication (Kokai) No. 2004-362953). The proposed polyester film, however, is not necessarily high in formability and difficult to apply to deep drawing. Other investigations are also under way for developing polyester film/polyamide laminate film to solve this problem (for instance, see Japanese Unexamined Patent Publication (Kokai) No. 2008-53133). This structure, however, requires complicated production steps, and in addition, cannot be high in formability.
For medical packaging, there are increased needs for packages cladded with metal foil such as aluminum foil that can prevent degradation of the contents, and development of metal foil with improved formability that can easily fit to the shape of contents is called for.
It could therefore be helpful to provide polyester film that exhibits high formability and high strength when used as material for packages that need processing steps using a die for deep drawing.